1. Field of the Invention
The present invention relates to a dispersant for dispersing nanoparticles in an epoxy resin, a method for dispersing nanoparticles using the dispersant, and a nanoparticle-containing thin film comprising the dispersant. More specifically, the present invention relates to a dispersant which includes a hydrophobic moiety having an affinity for capping ligands bound to the surfaces of nanoparticles and a hydrophilic moiety including a polyethylene glycol derivative structurally similar to an epoxide group; a method for dispersing nanoparticles by inserting the dispersant between capping ligands bound to the surfaces of the nanoparticles, to allow the surfaces of the nanoparticles to have an affinity for an epoxide group, thereby improving the dispersibility of the nanoparticles in a polymer matrix containing an epoxide group; and a nanoparticle-containing thin film comprising the dispersant.
2. Description of the Related Art
Quantum dots are nanometer-scale semiconductor materials, which exhibit quantum confinement effects. Quantum dots are used to fabricate a variety of electrical and optical devices due to their superior physical, chemical and electrical properties.
Since semiconductor nanoparticles exhibit size-dependent optical properties, they have received a great deal of attention in the fields of optical devices during the past decade. If a stable and hard nanoparticle thin film, comprising nanoparticles well dispersed in a matrix, having suitable optical properties were formed, a light-emitting device comprising the thin film would exhibit a constant luminescence level and high luminescence efficiency.
However, since nanoparticles have a strong tendency to aggregate, they cannot sufficiently exhibit their inherent characteristics. In recent years, attempts have been made to extend the applicability of nanoparticles in the fields of light-emitting devices. For example, approaches aimed at increasing the degree of dispersion of nanoparticles in various polymer matrices are gaining importance.
Epoxy resins containing an epoxide group, as polymer matrices, can be formed into optically transparent solids without any significant shrinkage when cured. Based on this characteristic, epoxy resins are promising candidate materials in various technological applications, such as encapsulation and optical devices.
However, nanoparticles do not tend to dissolve and/or disperse in epoxy resins, in part because epoxy resins have a relatively high viscosity and nanoparticles prepared by wet chemistry techniques, which are commonly employed to prepare nanoparticles, have short-chained capping ligands bound to their surfaces that contain no functional group having an affinity for an epoxide group.
Accordingly, a new type of dispersant, having a long chain and a functional group having an affinity for an epoxide group that is capable of sufficiently dispersing large concentrations of nanoparticles in a polymer matrix (e.g., an epoxy resin) containing an epoxide group without occurrence of aggregates or precipitates, is needed to fabricate light-emitting devices having improved optical properties.
Many processes to improve the dispersibility of nanoparticles have been proposed. For example, processes associated with surface modification to substitute capping ligands bound to the surface of nanoparticles with functional groups having an affinity for solvents or polymer matrices to be used have been attempted.
However, such substitution processes have many drawbacks and limitations. For example, the substitution processes cause damage to the surfaces of the nanoparticles, leading to poor efficiency, and are dependent upon the kind of capping ligands strongly bound to the surfaces of the nanoparticles because the functional groups must have a higher binding force than the capping ligands. Further, capping ligands separated from the nanoparticles are not readily removed.
To solve these problems, a method for modifying gold nanoparticles to make the nanoparticles water-soluble was recently reported (SCIENCE VOL 304 23 Apr. (2004)). According to this method, a dispersant having a hydrophilic moiety is inserted between hydrophobic capping ligands bound to the surface of the gold nanoparticles instead of by substituting the capping ligands with a functional group.
Although the method is successful in modifying gold nanoparticles to be water soluble, the report fails to describe a dispersant and a method for dispersing nanoparticles in highly viscous polymer matrices.
U.S. Patent Publication No. 2005/0074551 discloses water-soluble metal nanoparticles coated with a mixed monolayer, which comprises a capture coating component useful for the specific capture of materials and a shielding coating component for elimination of non-specific binding of materials to the capture component, and a process for preparing the water-soluble metal nanoparticles.
However, although this patent publication discloses a process for sufficiently dissolving metal nanoparticles (e.g., gold nanoparticles) in water so that the metal nanoparticles have an affinity for biomolecules (e.g., proteins) requiring aqueous environment for activity, there is no suggestion of a dispersant for sufficiently dispersing the nanoparticles in a polymer matrix, such as an epoxy resin, which would be useful in the fabrication of light-emitting devices.